In a new study published Thursday, scientists say they have solved the mystery of a neurological disease that has been killing bald eagles and other birds in the United States for more than 25 years. The disease appears to be caused by a toxin produced by a species of blue-green algae that grows on an invasive plant, a toxin that can occur in the presence of some contamination.
In 1994, there was a massive bald eagle kill in Arkansas. Before death came, predatory birds would lose their navigational abilities, crashing into trees or even losing their ability to fly. And when the scientists examined their brains post-mortem, they found distinct lesions and holes inside, making it appear that the brain had been eaten. Eventually, it was determined that the eagles had contracted the disease from the waterfowl they hunted and that they often displayed similar symptoms before dying. The condition became known as avian vacuolar myelinopathy (AVM).
Although scientists suspected that AVM was infectious in some way, the exact culprit remained unknown for years. Along the way, more AVM outbreaks occurred throughout the southeastern US near lakes and other freshwater sources. By the early 2000s, a clear connection had been made between the spread of an invasive aquatic plant called Hydrilla verticillata and AVM. By 2015, researchers from the University of Georgia provided evidence that a specific species of cyanobacteria – photosynthesizing bacteria – growing on this plant was responsible for AVM. The group named previously unknown species. Aetokthonos hydrillicola, translated from Greek and Latin to “slayer of eagles, living on hydrilla”.
Cyanobacteria are also called blue-green algae because of the color they give off when clustered in massive amounts (despite the nickname, they are not true algae, a vague term for many species of aquatic plants). They are often dangerous to animals, including people, due to the toxins they can produce. But when scientists at the University of Georgia and elsewhere tried to study TO. hydrillicola In isolation, they ran into a problem: the bacteria they grew in their lab were harmless to birds. They only appeared to be dangerous when growing on the plant.
In this new study, published On Thursday in the journal Science, scientists from the University of Georgia worked together with researchers from Germany and the Czech Republic to unravel the final pieces of the AVM puzzle. His work indicates that TO. hydrillicola It only produces the toxin that causes AVM when it is also around bromide, the negatively charged version of the element bromine.
Once they discovered this connection, the researchers were finally able to induce this toxin from their cultured laboratory samples. TO. hydrillicola and found that it could kill birds in the same way that AVM does in the wild. Genetic analysis of the bacteria also uncovered the specific pieces of DNA that allow it to produce the toxin. They dubbed their new discovery aetokthonotoxin (AETX), translated as “poison that kills the eagle.”
“We confirm that AETX is the causative agent of [vacuolar myelinopathy]”The researchers wrote in a summary of their findings.
While the exact method of killing behind AVM could be solved, questions remain. That is, where exactly does the bromide that fuels the production of this toxin come from, and why does AVM appear to be localized only to the US? Bromide exists naturally in many places, but it is also seen in many synthetic chemicals that could get into the aquatic environment. In particular, it can be found in certain herbicides used to control the spread of Hydrilla Plant around water treatment facilities and elsewhere. So it is possible that, by trying to get rid of a problem, we have contributed to creating a separate environmental crisis.
More research will be needed to confirm the role of these herbicides and other human-made sources of bromide in causing AVM outbreaks, but the authors already recommend that they not be used to control Hydrilla stocks for longer. Because this toxin can accumulate in animals other than birds, such as reptiles, fish, and amphibians, it is also possible that it can make mammals, including humans, sick.
Toxic blooms caused by algae (including blue-green algae) have already become more intense around the world over the past few decades, and rising temperatures will likely only make matters worse. And while AVM outbreaks have only been seen in four states to date, the scale of the problem is likely larger than has been officially documented.
Revealing the identity of this eagle killer is definitely cause for celebration, but stopping him will be a whole new challenge.